The present invention relates to a compressor casing for a turbocharger and an assembly thereof.
Generally as shown in FIG. 1, a turbocharger utilizes exhaust gas 2 from an engine 1 to drive a turbine 3 which in turn drives a compressor 4 coupled through a common shaft with the turbine 3, thereby charging compressed air 5 into the engine 1.
The compressor 4 comprises, as shown in FIG. 2, an integrally cast compressor casing 10 which defines an air inlet port 6, an air intake passage 7 extending from the port 6, a diffuser 8 extending radially from an outlet end of the passage 7 and a scroll 9 extending to surround over outer surfaces of the diffuser 8 and passage 7, all of which are communicated with each other in the order named. An impeller 11 is rotatably accommodated in the passage 7 and its rotation by the turbine 3 will cause the air 5 to be sucked through the port 6 into the passage 7, increased in pressure while flowing through the diffuser 8 and charged through the scroll 9 into the engine 1.
With the compressor 4 described above, in a low flow rate range where the volume of intake air 5 is small, the air 5, increased in pressure by the diffuser 8, may flow back or reverse, causing low-noise vibrations called surging. In order to prevent such surging or to displace the surge range to a lower flow rate, range, heretofore a ring-shaped space 12 is formed adjacent to the port 6 upon casting of the casing 10 so as to open to the port 6 and surround the passage 7. A ring-shaped groove 13 or a plurality of through holes are machined through the casing 10 such that the groove 13 extends from the passage 7 to an innermost portion of the space 12, thereby defining an air passage 14 for the discharge therethrough of reversing air 5.
Reference numeral 15 denotes reinforcing ribs arranged circumferentially of the space 12. Upon driving of the engine with a low volume of intake air, the reversing air 5 is permitted to flow out of the passage 14 and upon driving of the engine with a high volume of intake air, the air 5 is charged also through the passage 14 into the engine.
The above-mentioned conventional compressor casing for a turbocharger have problems.
The effect of discharging the reversing air 5 through the passage 14 in the low-intake-air-volume driving so as to displace the surging-occuring range to a lower flow rate range is insufficient since the passage 14 is open to the air inlet port 6 and the intake air flow through the port 6 into the passage 7 suppresses the discharge of the air 5 out of the passage 14. Consequently, the desired capability of displacing the surging-occuring range to a lower flow rate range is insufficiently attained.
Fabrication cost for the compressor casing 10 is high since the casing 10 is integrally cast which has the ring-shaped space 12 defining the air passage 14 and is structurally complicated.
Machining is very much cumbersome since the groove 13 or a plurality of through holes must be machined for communication of the passage 7 with the innermost portion of the space 12.
In view of the above, a primary object of the present invention is to provide a turbocharger compressor casing and its assembly with enhanced ability of displacing the surging-occurring range to a lower flow rate range and with an air passage capable of lowering the casting cost and facilitating the machining.